Desalination: Difference between revisions
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Author: [[user:Hschwarz|Hans-Jürgen Schwarz]] | Author: [[user:Hschwarz|Hans-Jürgen Schwarz]] | ||
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== Abstract == | == Abstract == | ||
If there is an increased salt load, the reduction of the salts is a method to deal with. Salt reduction measures include the possibilities of cpompress desalination, alone or in combination with other methods. Reducing the salt content of an object in a water bath as well as by electric current supported means are other options. When choosing the method the protection of the object has always the first priority. By appropriate investigations the success of a measure has to be ensured. | |||
== Introduction== | == Introduction== | ||
Desalination | Desalination is the removal of salts and salt-forming ions out of the pore structure of porous materials such as natural rocks (sandstones, limestones, tuffs, etc.), brick or terra cotta, plaster and wall paintings, in situ at the object or on moveable objects in the workshop. | ||
The most commonly encountered salts are sulphates ( | The most commonly encountered salts are sulphates (Gipsum CaSO<sub><font size="1">4</font></sub><font size="1">•</font>2H<sub><font size="1">2</font></sub>O, Mirabilite (Thenardite) Na<sub><font size="1">2</font></sub>SO<font size="1">4</font>•10H<sub><font size="1">2</font></sub>O (Na<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font></sub>), magnesium sulphate (MgSO<font size="1">4</font>•7H<sub><font size="1">2</font></sub>O u.a), chlorides (NaCl u.a.) und nitrates (Nitrokalit KNO<sub><font size="1">3</font></sub> u.a.). In individual cases, differnet salts can exist side by side, and a variety of salt-forming ions in the pore solution. | ||
Salts | Salts are damaging the structure of porous materials and lead to dusting with sometimes great material loss. The amout of destruction and its appearence depend on the kind of crystallizing salts, the concentration of the salt solutions and the environment conditions. Particularly damaging are climate fluctuations around the[[deliquescence|Deliquescence Humidity]]point of the salts. In addition, water-soluble salts have an ingfluence on conservation measures such as strengthening, waterproofing, painting or plastering or make even such action impossible. For these reasons looking to the success and the durability of a measure the reduction of the salt content is an indispensable prerequisite. | ||
The desalination/ salt reduction can be | The desalination/ salt reduction can be done in several ways <bib id="Sawdy.etal:2006" />. The use of plaster/ slurries with salt-contaminated objects <bib id="Auras:2008" /> is described [[Plaster/Slurries|elsewhere]]. | ||
== [[Water Bath Desalination]] == | == [[Water Bath Desalination]] == | ||
This method is only | This method is practicable only for objects that can be transported to a workshop, usually for sculptures and objects that can be removed for a restoration.<bib id=Franzen.etal:2008/> | ||
The | The object contaminated with salts is placed in a bath of cold or slightly warm water. In doing so the water can be desalinated and circulated to enhance the desalination process. Easier but not so effective the water is exchanged from time to time. The efficiency of desalination is monitored by measuring the conductivity of the water bath. | ||
Degree and speed of | Degree and speed of desalination depends on the size of the object, the properties of the material(e.g. fine pores or coarsely porous stone), the type and amount of salts and salt-forming ions and their distribution in the pores. Salts concentrated near the surface are removed faster than those from deeper areas. Generally life-size figures need of application from several weeks to several months. | ||
If the material is suitable, desalination in a water bath has a good chance of success. Specific risk factors are: | |||
*the | *the total penetration of the entire pore structure with water, risk for the paint layers; | ||
*advanced degree of destruction: flaking of brittle surfaces; | *advanced degree of destruction: flaking of the brittle surfaces; | ||
*salts with several | *salts with several hydrat phases: hydration may be triggered, leading to a loss of substance. | ||
A pre-consolidation of brittle surfaces with | A pre-consolidation of brittle surfaces with suitable strengthening agents may be possible (for example: silicic acid esters). The desalination can be delayed in some cases considerably. | ||
== [[Desalination | == [[Desalination compresses]] == | ||
Desalination | Desalination with compresses relies on the principle of solving salts in water and transport it from the salt-loaded, porous, mineral building materials into the compress. The transport of the salts in solution can take place both by their own movement as well as by the moving fluid. The motion of a fluid is usually troggered by a moisture gradient (capillary) or by temperature, density and pressure gradients (convection). | ||
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Dagegen führen Konzentrationsgradienten zur Eigenbewegung der Salzionen (Diffusion). Der Kapillartransport (Advektion) wird durch das Porengefüge des Baustoffes bestimmt und kann in einfacher Weise durch den Wasseraufnahmekoeffizienten charakterisiert werden <bib id="Heritage.etal:2008"/>. Die Transportrichtung der Ionen verläuft entsprechend dem Feuchtegradienten vom feuchteren zum trockeneren Bereich. Die treibende Kraft für einen Ionentransport durch Diffusion ist ein Konzentrationsgefälle. Die Ionen diffundieren entsprechend dem Konzentrationsgradienten von der höheren zur niedrigeren Konzentration. Diffusion findet auch als Oberflächendiffusion an den Grenzflächen statt. Der konvektive Transport wird durch Druck-, Dichte- und Temperaturdifferenzen hervorgerufen und kann in einfacher Weise durch die Wasserdurchlässigkeit und andere Versuche überprüft werden. Dieser Transportprozess tritt bevorzugt in größeren Poren (<nowiki>></nowiki> 0,1 mm), Rissen und Hohlstellen auf. | Dagegen führen Konzentrationsgradienten zur Eigenbewegung der Salzionen (Diffusion). Der Kapillartransport (Advektion) wird durch das Porengefüge des Baustoffes bestimmt und kann in einfacher Weise durch den Wasseraufnahmekoeffizienten charakterisiert werden <bib id="Heritage.etal:2008"/>. Die Transportrichtung der Ionen verläuft entsprechend dem Feuchtegradienten vom feuchteren zum trockeneren Bereich. Die treibende Kraft für einen Ionentransport durch Diffusion ist ein Konzentrationsgefälle. Die Ionen diffundieren entsprechend dem Konzentrationsgradienten von der höheren zur niedrigeren Konzentration. Diffusion findet auch als Oberflächendiffusion an den Grenzflächen statt. Der konvektive Transport wird durch Druck-, Dichte- und Temperaturdifferenzen hervorgerufen und kann in einfacher Weise durch die Wasserdurchlässigkeit und andere Versuche überprüft werden. Dieser Transportprozess tritt bevorzugt in größeren Poren (<nowiki>></nowiki> 0,1 mm), Rissen und Hohlstellen auf. | ||
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''Zweckmäßigkeit der Entsalzung'' | ''Zweckmäßigkeit der Entsalzung'' | ||
Es ist zu prüfen, ob eine Entsalzung überhaupt erfolgreich durchführbar ist. Erfolg verspricht eine Entsalzung nur dann, wenn die Salze hauptsächlich in der Oberflächenzone auftreten. Gleichmäßige Versalzungen von annähernd 1 Gew.% über die | Es ist zu prüfen, ob eine Entsalzung überhaupt erfolgreich durchführbar ist. Erfolg verspricht eine Entsalzung nur dann, wenn die Salze hauptsächlich in der Oberflächenzone auftreten. Gleichmäßige Versalzungen von annähernd 1 Gew.% über die gesamte Mauerwerksstärke, wie sie häufig bei Nitratversalzung auftreten, können nicht mit Aussicht auf Erfolg behandelt werden. Hier sind andere Lösungen, z. B. Nutzungsänderungen, anzustreben. | ||
''Schonung der Originalsubstanz'' | ''Schonung der Originalsubstanz'' | ||
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Geringere Konzentrationen von Salzen gefährden nicht den Erfolg von Konservierungsmaßnahmen. Entsalzungen können dann entfallen. Allerdings läßt sich nur ein materialspezifischer Grenzwertbereich einer Salzbelastung angeben, da Porosität, Porenradienverteilung und Klima eine entscheidende Rolle spielen. Hier ist im Einzelfall nach Gutachten zu entscheiden. Es ist bekannt, dass Salze in höheren Konzentrationen eine Festigung mit Kieselsäueester oder eine Hydrophobierung beeinträchtigen. Auch die Dauerhaftigkeit der Maßnahme wird stark eingeschränkt. | Geringere Konzentrationen von Salzen gefährden nicht den Erfolg von Konservierungsmaßnahmen. Entsalzungen können dann entfallen. Allerdings läßt sich nur ein materialspezifischer Grenzwertbereich einer Salzbelastung angeben, da Porosität, Porenradienverteilung und Klima eine entscheidende Rolle spielen. Hier ist im Einzelfall nach Gutachten zu entscheiden. Es ist bekannt, dass Salze in höheren Konzentrationen eine Festigung mit Kieselsäueester oder eine Hydrophobierung beeinträchtigen. Auch die Dauerhaftigkeit der Maßnahme wird stark eingeschränkt. | ||
--> | --> | ||
== | == Success control== | ||
The salt content in the stone, plaster or brick should be measured before and after the application to check the success of desalination. | |||
On desalination to buildings, there are a few experiences. Basically, one can expect a satisfactory desalination of any kind only if the salts are concentrated near the surface at 1-2 cm depth. Either with compresses or with electrochemical methods the desalination reaches only a few centimeters of the materials. | |||
Example: | Example: With a bentonite / sand / cellulose compress applied twice in the splash zone of the "Nürnberger Tor" in Forchheim a NaCl contamination was removed up to 90%. This was due to the fact, that the salinity was confined to the uppermost centimeters. | ||
== Literature == | == Literature == | ||
Revision as of 07:28, 5 October 2011
<bibimport />
Author: Hans-Jürgen Schwarz
back to Measures
Abstract
If there is an increased salt load, the reduction of the salts is a method to deal with. Salt reduction measures include the possibilities of cpompress desalination, alone or in combination with other methods. Reducing the salt content of an object in a water bath as well as by electric current supported means are other options. When choosing the method the protection of the object has always the first priority. By appropriate investigations the success of a measure has to be ensured.
Introduction
Desalination is the removal of salts and salt-forming ions out of the pore structure of porous materials such as natural rocks (sandstones, limestones, tuffs, etc.), brick or terra cotta, plaster and wall paintings, in situ at the object or on moveable objects in the workshop.
The most commonly encountered salts are sulphates (Gipsum CaSO4•2H2O, Mirabilite (Thenardite) Na2SO4•10H2O (Na2SO4), magnesium sulphate (MgSO4•7H2O u.a), chlorides (NaCl u.a.) und nitrates (Nitrokalit KNO3 u.a.). In individual cases, differnet salts can exist side by side, and a variety of salt-forming ions in the pore solution.
Salts are damaging the structure of porous materials and lead to dusting with sometimes great material loss. The amout of destruction and its appearence depend on the kind of crystallizing salts, the concentration of the salt solutions and the environment conditions. Particularly damaging are climate fluctuations around theDeliquescence Humiditypoint of the salts. In addition, water-soluble salts have an ingfluence on conservation measures such as strengthening, waterproofing, painting or plastering or make even such action impossible. For these reasons looking to the success and the durability of a measure the reduction of the salt content is an indispensable prerequisite.
The desalination/ salt reduction can be done in several ways [Sawdy.etal:2006]Title: Desalination—rubbing salt into the wound?
Author: Sawdy, Alison; Heritage, Adrian
. The use of plaster/ slurries with salt-contaminated objects [Auras:2008]Title: Poultices and mortars for salt contaminated masonry and stone objects
Author: Auras, Michael is described elsewhere.
Water Bath Desalination
This method is practicable only for objects that can be transported to a workshop, usually for sculptures and objects that can be removed for a restoration.[Franzen.etal:2008]Title: Water bath desalination of sandstone objects
Author: Franzen, Christoph; Hoferick, Frank; Laue, Steffen; Siedel, Heiner
The object contaminated with salts is placed in a bath of cold or slightly warm water. In doing so the water can be desalinated and circulated to enhance the desalination process. Easier but not so effective the water is exchanged from time to time. The efficiency of desalination is monitored by measuring the conductivity of the water bath.
Degree and speed of desalination depends on the size of the object, the properties of the material(e.g. fine pores or coarsely porous stone), the type and amount of salts and salt-forming ions and their distribution in the pores. Salts concentrated near the surface are removed faster than those from deeper areas. Generally life-size figures need of application from several weeks to several months.
If the material is suitable, desalination in a water bath has a good chance of success. Specific risk factors are:
- the total penetration of the entire pore structure with water, risk for the paint layers;
- advanced degree of destruction: flaking of the brittle surfaces;
- salts with several hydrat phases: hydration may be triggered, leading to a loss of substance.
A pre-consolidation of brittle surfaces with suitable strengthening agents may be possible (for example: silicic acid esters). The desalination can be delayed in some cases considerably.
Desalination compresses
Desalination with compresses relies on the principle of solving salts in water and transport it from the salt-loaded, porous, mineral building materials into the compress. The transport of the salts in solution can take place both by their own movement as well as by the moving fluid. The motion of a fluid is usually troggered by a moisture gradient (capillary) or by temperature, density and pressure gradients (convection).
Success control
The salt content in the stone, plaster or brick should be measured before and after the application to check the success of desalination.
On desalination to buildings, there are a few experiences. Basically, one can expect a satisfactory desalination of any kind only if the salts are concentrated near the surface at 1-2 cm depth. Either with compresses or with electrochemical methods the desalination reaches only a few centimeters of the materials.
Example: With a bentonite / sand / cellulose compress applied twice in the splash zone of the "Nürnberger Tor" in Forchheim a NaCl contamination was removed up to 90%. This was due to the fact, that the salinity was confined to the uppermost centimeters.
Literature
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